1. Field of the Invention
This invention relates to artificial cardiac pacemaker and more particularly, to a cardiac pacemaker which may or may not be implantable within the human body and which will respond to the changing needs of the body but will not compete with the natural cardiac electrical activity of any kind.
2. State of the Prior Art
The implantable cardiac pacemaker, shown in U.S. Pat. No. 3,057,356 permits innocuous, painless, long-term cardiac stimulation at low power levels by utilizing a small, completely implanted transistorized and battery-operated pacemaker connected via flexible electrode wires directly to the cardiac muscle. Such an asynchronous pacemaker or pacer, while providing fixed-rate stimulation not automatically changed in accordance with the body's needs, has proven effective in alleviating the symptoms of complete heart block. An asynchronous pacer, however, has the possible disadvantage of competing with the natural, physiological cardiac pacemaker during episodes of normal sinus conduction.
A synchronous or P-wave pacer, shown in U.S. Pat. No. 3,253,596, has been proposed for producing a stimulus following each P-wave or atrial beat. When the body signals a need for increased heart rate, as indicated by an increasing atrial beat, the synchronous pacer responds with an increased ventricular stimulation rate. However, the function of the known synchronous pacer is not responsive to irregular ventricular ectopic activity and may compete against such beats. Thus, while the synchronous pacer will not compete against normally conducted beats, it can compete against ectopic or abnormally conducted beats. Any competition between the natural and the artificial pacer may be undesirable because such competition may possibly lead to incidents of tachycardia or even fibrillation.
There has also been developed a ventricular inhibited or demand-type pacer, shown in U.S. Pat. No. 3,478,746, in which case the artificial stimuli are initiated only when required and subsequently can be eliminated when the heart returns to sinus rhythm above a predetermined base rate. The demand pacer solves the problem arising in asynchronous pacer by inhibiting itself in the presence of ventricular activity but coming "on line" and filling in missed heart beats in the absence of ventricular activity after a base time interval. When the demand pacer comes "on line", it operates as an asynchronous pacer, the rate of which is not responsive to atrial activity.
To cope with the problems of the synchronous pacer, a further pacer has been developed, shown in U.S. Pat. No. 3,648,707, which stimulates the heart asynchronously in the absence of cardiac electrical activity of any kind, inhibits itself and becomes completely dormant for a suitable interval in the presence of a single ventricular beat, ectopic or conducted from the atrium, and stimulates the heart synchronously in the presence of atrial activity not accompanied by arrythmic ventricular activity.
To the best of my knowledge, all versions of the atrial synchronous pacer that have been publicly disclosed and/or marketed have stimulated the heart synchronously in the presence of sensed atrial activity in a certain range extending from the asynchronous lower rate of the pacer circuit up to a maximum upper rate. Any natural P-wave rate exceeding the upper rate results in a sudden change of patient's heart beat and a concurrent sudden loss of cardiac output. Presumably, the patient's atrial rate had risen to the upper limit due to exertion or stress requiring increased cardiac output, and the sudden change in cardiac output could cause that patient to suddenly become faint and possibly be endangered in view of the circumstances causing the atrial rate to rise.
This particular trait of the synchronous pacer was deliberately designed in because a physiological condition of the cardiac muscle left it vulnerable to irritation by external electrical stimulae during a predetermined time period following a complete depolarization of the heart muscle. For example, if the depolarization of the heart is caused by a pacer pulse stimulus synchronously following a sensed atrial depolarization, the heart muscle must repolarize over a predetermined time interval, named the T-wave interval. If a second atrial depolarization is sensed too soon following the first depolarization, the pacer stimulus, if applied to the heart during the vulnerable period might conceivably elicit bursts of tachycardia of fibrillation which are undesirable and may even lead to a fatal sequence of arrythmias.
With this fear in mind, advantage was taken of the fact that sense amplifier circuits in synchronous and demand pacer circuits have a built-in refractory period in which they are insensitive to any incoming signal following too closely a previously sensed signal in order to prevent their sensing the pacer's own stimulus. This refractory period of the sense amplifiers in synchronous pacers was allowed to establish the maximum allowable rate at which the pacer would synchronously follow the atrial depolarizations. As described in U.S. Pat. No. 3,648,707 the refractory period of the atrial sense amplifier might be set at 500 milliseconds, (conforming to a maximum synchronous rate of 120 beats per minute), so that if the atrial depolarizations followed each other by less than 500 milliseconds, and a second P-wave signal would arrive at the P-wave sense amplifier while it was still refractory, only every other atrial depolarization would be sensed and the pacer rate would be one-half the atrial rate. Through the same mechanism, if the atrial rate increases even more, and the atrial depolarization occur within 500 milliseconds, only one would be sensed, and the pacer would operate at one-third the atrial rate.
Alternatively, pacer circuits have been designed to revert to an asynchronous base rate mode once the atrial depolarizations occur at a rate exceeding the upper limit. The asynchronous mode rate is the lower limit at which the pacer circuit functions in the absence of any heart activity. In this case circuit designers have again taken advantage of a pre-existing pacer circuit to effect this operation.
As mentioned earlier, the sudden transition from a high pacing rate to a lower rate, while physiologically beneficial in the sense that cardiac stimulation during the vulnerable period is prevented, causes sudden symptomatic interruption in cardiac output and that may indirectly cause physical harm to the patient.